I was intrigued by this and found an interesting (and not too technical) discussion of fish eyesight at http://www.petsforum.com/mas/masart80.htm. Apparently, a fish’s cornea doesn’t do much of anything to help focus the light (because the corneal tissue has about the same refractive index as water), so it’s all done by “the lens of the fish, which has the highest affective refractive index of all vertebrates (1·65).” I guess that means conventional laser surgery wouldn’t work on fish. If Bubbles is myopic, maybe it really will need those “miniature goggles”…
Well, you could presumably use laser surgery to re-shape the fish’s lens, if the surgeon doesn’t go into spontaneous giggle fits throughout the operation.
Umm, Chronos, did you run this answer by an ophthalmologist or optometrist? Cuz I asked my eye doctor this very question years ago and got a very different answer. His explanation was that water washes away the tear layer, changing the optics of the cornea. I think he meant by scattering light, but I may have filled that in myself.
Frankly, that explanation makes more intuitive sense to me than the angle-of-refraction one. But I must confess I’m neither an expert, nor do I have any citable authority at hand. Hence my question.
Um, although the article and the messages add some reasons, I think the biggest reason is also the simplest. When you’re in air, there is no more pressure on your eye than what you’ve grown up with. Underwater, however, the pressure is greater, so when you open your eye, your eyeball deforms by a tiny amount, yet enough to blur your vision a little. For example, the lightest touch to your eyelid will blur your vision. Goggles simply provide a little air to reduce the pressure against the lense.
I’m having trouble with the idea that goggles work because they’re flat. I wish I had curved goggles to test this out, but I have to believe that curved goggles would work just as well. Weren’t those round windows in the ‘20,000 leagues under the sea’ ride curved? Also, I seem to recall seeing a one-person submarine on TV where the entire nose of the sub, where the rider’s head is, was made of glass. The effect was supposed to be similar to flying.
Guy with an optiics degree weighing in here – Chronos has it straight. Index of refraction is the key. I never heard this thing about tears before – but tears are basically salinbe, and the index is very nearly that of water. Pressure changes (as long as the water isn’t really deep) are negligible effects, too. Index of refraction change is far and away the biggest effect.
Besides wearing a set of goggles, you could also wear corrective glasses underwater. I have a reprint from an old optics journal about this. The problem is, it requires a really big index difference. If you tried to make it out of glass, it wouldn’t work very well. People have made such lenses with air as the main component (held in place by glass or plastic), but they’re still oddly-shaped. Besides, opening their eyes underwater bothers a lot of people – they’d rather wear air-filled goggles, anyway.
As far as fish go, I have to mention the Asian fish that swim with eyes half above and half below the water, since they hunt prey that lives in air. Their eyes are essentially double eyes, withb different construction above and below to allow for focusing both in air and in water.
Cal, are you referring to the anablep (the “four-eyed fish”)? If so, I believe it’s a native of Central and South America.
You might be interested in this page: http://www.schools.net.au/edu/lesson_ideas/optics/optics_wksht4_p1.html. About two-thirds of the way down, there’s a description and diagram of the anablep’s eye. What I found amazing is that even though each eye has, in effect, two pupils and two retinas, there’s only one oval-shaped lens. Light from above the water’s surface passes through the flatter section of the lens while light from below passes through the more rounded part. I know very little about optics, but it seems like quite a remarkable design.
Any curved interface between materials of different indices will act as a lens, including curved submarine windows or swim goggles. Neither of those has very much curvature, though, so they’re very weak lenses, and don’t make much practical difference.
As for the tear layer, I find that when I get out of the water, I can almost immediately see nearly as clearly as before I got in. My eyes are a little uncomfortable until the tears are replenished, but I can see through them.
On St1d’s suggestion about pressure - definately not the answer. Chronos has it right. Scuba divers who wear masks subject their eyes to the same pressure as non-mask wearers, except the pressure is air pressure, not water pressure. If the pressure really mattered, it would get progressively more difficult to see as you went deeper, and just submerging your barely under the surface wouldn’t matter. Diving to 100 ft would be nearly blind. The reason that pressure does not affect the shape of the eyeball appreciably is that there is no gas pocket within the eyeball. Water (and most liquids and solids) do not compact appreciably under the pressures we are talking about here. You put your finger in your eye and it will deform because the liquids and solids will squash around. But if the pressure is even (as in water or air pressure) then there will be no discomfort, and no changing of the shape of the eyeball.
I don’t know if I trust this source completely (I thought the surgery was called radial keratotomy), but I do remember the reports about the gentleman losing his sight due to the deformation of his eyeball. So, even though Fishhead may be right generally, particular people may experience some eyeball warp-age due to pressure.
Li’l Dickie, your author refers to this “radical keratotomy” as a laser surgery, so it ain’t RK, but I don’t know what the hell he means.
Certainly pressure is capable of distorting the eyeball, and certainly those with deliberately thinned (and therefore weakened) tissues will experience more distortion. However, this low-pressure distortion does not show that high pressure will cause any distortion sufficient to cause poor vision, even in those who have had ocular surgery.
Actually, it probably wasn’t the low pressure per se which caused his eyesight to fail. It’s the low partial pressure of oxygen. Loss of visual acuity is a symptom of oxygen deprivation.
Fair enough. Let me just say that I’m not disputing Chronos’ answer. I believe him to be correct. There was even a time in my swimming “career” when it seemed that I could see better underwater than I could out of water with my glasses on (I’m nearsighted). I chalked it up to the difference in the index of refraction between water and air. It no longer seems that way now, but my vision has worsened somewhat in the meantime. Or it might have been my imagination.
For all I know, you’re right, but that’s not how it was reported in the (popular) media. And I think the guy was carrying O2, but I could be wrong.
I bet it did mean radial kerototomy. Some confusion either on the patient’s part, misremembering, or confusion on the second-hand author’s part could easily confuse “radial” with “radical”. As for being a laser surgery, there are various techniques for radial kerototomy. First came the knife type (scalpel), then came laser surgery, using lasers to perform the same type of cutting. The newest type is LASEK (sp?), which is a laser surgery but instead peels a layer of the cornea and then cuts the middle sections, before putting the layer back.
As for pressure, submariners are under pressure and have no difficulty seeing. Same as SCUBA divers. It’s not pressure.
Dickie, who says the media actually get things right?
A curved surface of glass or plastic that remains uniformly thick will not act as a lens nearly as much as the glass or plastic where the two surfaces follow different curves.
Not me, that’s for sure. I’m not trying to put any weight behind the reports. I actually threw the parenthesis in there to qualify them as conditionally (un)trustworthy, as opposed to, say, the American Journal of Ophthalmology or the NEJM, which are much more likely to be correct.
The article is a column “Written by: Dr. Gifford-Jones”; I don’t know what kind of doctor he is, but I assume he knows the difference between radial and radical, and between a laser and a microkeratome. FYI, RK is never, ever, ever performed using a laser. Ever.
The laser surgeries are PRK, LASIK, and LASEK. LASEK, the most recent type, is the only one in which the laser is used for cutting (as opposed to ablation, which all three use lasers for).
Do any of the surgeries for vision correction operate on the lens of the eye, as opposed to the cornea?
The reason I ask is that in a previous mesage, Chronos stated that “you could presumably use laser surgery to re-shape the fish’s lens”. I was surprised at this because I had thought that vision correction surgery only altered the corneas. (And cataract surgery generally just removes the clouded lens and repaces it with an artificial one, doesn’t it?)
